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    Acute effects of combined and isolated caffeine and theanine supplementation on physical and cognitive performance in competitive athletes: a randomized, double-blind, placebo-controlled crossover study
    (Frontiers Media Sa, 2026) Tuncer, Selin Yildirim; Ozdenk, Serhat; Yildirim, Ulas Can; Erkan, Dilara; Sari, Cengizhan; Gundem, Mehmet Can; Sar, Halit
    Introduction Modern athletic performance is driven not only by physical capacity but also by rapid decision-making, attentional control, and visuomotor coordination. Evidence regarding the acute effects of caffeine (CAF), L-theanine (TEA), and their combination remains inconsistent, particularly with respect to their combined influence on physical and cognitive performance in athletic populations. This study examined the acute effects of isolated and combined CAF and TEA supplementation on maximal strength, intermittent aerobic endurance, and eye-hand coordination in competitive athletes. It was hypothesized that the combined ingestion of CAF and TEA would differentially affect physical performance and eye-hand coordination outcomes compared with isolated CAF or TEA intake.Methods Twenty trained athletes completed four randomized, double-blind, placebo-controlled crossover conditions: CAF (3 mgkg-1), TEA (200 mg), CAF+TEA (COM), and placebo (CON), with >= 72 h washout. Outcomes included isometric leg, back, and handgrip strength; Yo-Yo Intermittent Endurance Test Level 1 performance; and visuomotor coordination (CogniFit (R)). Data were analyzed using repeated-measures ANOVA with Bonferroni corrections (alpha = 0.05).Results Condition effects were found for leg strength (p = 0.004, eta p2 = 0.24) and back strength (p = 0.008, eta p2 = 0.19). In the COM condition, no additional or synergistic effect on maximal strength was observed. Additionally, no significant difference was found between the conditions in aerobic endurance, maximum isometric handgrip strength, and hand-eye coordination results. Caffeine ingestion did not differ from placebo for any strength outcome under the present conditions. Finally, acute ingestion of TEA was associated with reduced maximal isometric leg and back strength compared with CAF and CON.Discussion Findings challenge the prevailing assumption of CAF + TEA synergy and underscore the need for task-specific interpretation of co-supplementation strategies. Future studies should evaluate dose-response interactions, habitual caffeine intake, sex-specific responses, and broader cognitive domains beyond visuomotor control. The randomized controlled trial was registered at ClinicalTrials.gov, under the registration number NCT07268573.Clinical Trial Registration ClinicalTrials.gov, NCT07268573.
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    Comparative effects of caffeine and paraxanthine on rowing performance and sleep quality: a randomized crossover study
    (Taylor & Francis Inc, 2026) Bingol Diedhiou, Azize; Yildirim, Ulas Can; Ozdenk, Serhat; Erkan, Dilara; Karakulak, Izzet; Yildirim Tuncer, Selin; Turgut, Murat
    BackgroundAlthough caffeine is widely used in athletes due to its ergogenic effects, the effects of its main metabolite, paraxanthine, on performance and sleep have not been adequately investigated.PurposeThis study aimed to comparatively investigate the effects of caffeine and its main metabolite paraxanthine on rowing performance and sleep quality.MethodsThe study was designed as a randomized, double-blind, crossover study and included 14 male university-level rowers (21.6 +/- 1.9 age; 2.2 +/- 1 years of rowing experience). The participants participated in 2000-m rowing ergometer time trials under four different supplementation conditions (caffeine + paraxanthine, caffeine + placebo, paraxanthine + placebo, and placebo with 200 mg each). Performance data (completion time, mean power, and heart rate), sleep quality, and daytime sleepiness were assessed by subjective scales. The data were analyzed by repeated-measures analysis of variance (ANOVA) and Bonferroni post-hoc tests.ResultsCompared with the placebo, the combined caffeine + paraxanthine condition was associated with faster 2000-m performance and higher mean power output (p = 0.044; Cohen's d = 0.30). Caffeine alone and paraxanthine alone did not show clear evidence of performance improvement in this sample, although estimates favored both conditions versus placebo. Conditions containing caffeine were associated with poorer subjective sleep quality, whereas paraxanthine alone showed more favorable sleep-related outcomes.ConclusionThese results indicate that the combined ingestion of caffeine and paraxanthine elicited the most pronounced performance benefits, while paraxanthine alone did not demonstrate clear standalone ergogenic efficacy at the administered dose. However, paraxanthine was associated with better subjective sleep outcomes compared to caffeine, suggesting that its potential value may be related more to tolerability rather than superior performance enhancement, particularly for athletes training in the evening. Study limitations, including the small sample size and lack of objective sleep measures, should be considered when interpreting the results.Practical applicationsParaxanthine may represent a practical option for athletes who prioritize sleep quality or experience sensitivity to caffeine-related sleep disturbances, although further dose-response studies are required to clarify its ergogenic potential.